Focusing on the transamination reaction between ketoacids and amino acids — that nowadays is catalysed by transaminase enzymes with PLP as the cofactor — the researchers initially surveyed metals due to their known activity, biological relevance and environmental abundance, in the presence or absence of the coenzymes PL and PLP (pictured). They used high-purity reagents and identified the role of the buffer, which is often not innocent in a reaction but necessary to ensure reproducibility and control the pH, as well as temperature. They find that Fe3+ and Al3+ with PL showed higher rate accelerations compared to metal-only or cofactor-only catalysis, or other metal–coenzyme complexes, concluding that the PL scaffold amplified the metal’s reactivity. The metal–PL co-catalysis mechanism was studied in detail for the Al–PL pair, via 1H nuclear magnetic resonance spectroscopy, kinetic isotopic effects, and by determining the resting state as well as the rate-determining step (RDS). They found that the active catalyst is a 1:1 Al–PL complex and that the metal presence lowers the pKa of the metal–PL–amino acid complex and slows the imine hydrolysis, which becomes the RDS, while imine formation increases the concentration of the active catalyst. This is different from what is observed for the PL-only or metal-only catalysed reaction, or for the enzymatic reaction — in which multiple RDSs are involved — and identifies a possible intermediate evolutionary stage where the metal centre could have preorganized the metal–PL–peptide system. However, metal chelation can hinder the product release, and this is the speculated reason for the evolved metal-free transamination in enzymes.
The synergistic effect of PL and abundant metals on the transamination of amino acids and keto acids is an example of cofactors acting in metabolic reactions in the absence of enzymes, and provides insights into how protometabolism may have started to emerge.
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